Corrosion tests of high temperature alloys in impure helium

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1 Proceedings of the HTR 214 Corrosion tests of high temperture lloys in impure helium Jn Berk 1,2, Jn Klivodov 1, Monik Vilemov 3, Zuzn Skoumlov 4, Petr Brec 4 1 Resrch Centre Rez Ltd. Husinec - Řež, Hlvní 13, 2568 Řež, Czech Repulic phone: , jn.erk@cvrez.cz 2 Institute of Chemicl Technology in Prgue, Technická 195, Prgue 6, Czech Repulic 3 Institute of Plsm Physics AS CR, v.v.i., Z Slovnkou 1782/3, 182 Prgue 8, Czech Repulic 4 UJV Rez Inc, Husinec-Řež, Hlvní 13, 2568 Řež, Czech Repulic. Astrct Czech reserch orgniztions tke prt severl projects concerning technologies nd mterils for dvnced gs cooled rectors, s n exmple interntionl project ARCHER supported y EU within FP7, lso severl ntionl projects supported y Technology Agency of the Czech Repulic re solved in coopertion with industril nd reserch orgniztion. Within these projects the mteril testing progrm is performed. The results presented in these pper concerning high temperture corrosion nd degrdtion of lloys (8 H, SS 316 nd P91) in helium contining minor impurities (H 2, CO, CH 4, H 2 O) t tempertures up to 76 C. After corrosion tests (up to 15 hours) the specimens ws investigted y severl methods (grvimetry, SEM-EDX, opticl microscopy, hrdness nd micro-hrdness testing etc.). I. INTRODUCTION Czech reserch nd industril orgniztions re involved in projects concerning mterils for high temperture pplictions, mong others lso for helium cooled high temperture rectors. As n exmples the ARCHER project (Advnced High- Temperture Rectors for Cogenertion of Het nd Electricity R&D) [1] supported within Frme Progrm 7 EU could e nmed. Other project re supported y Ministry of Industry nd Trde of Czech Repulic nd Technologicl Agency of Czech Repulic. Within the projects corrosion nd degrdtion of high temperture lloys especilly ferritic steel P91, ustenitic steel 316 SS nd lloy 8H were tested. These lloys re on the list of metllic mterils considered for VHTR components [2]. The purpose of testing ws not only to verify the properties of tested mterils in VHTR helium coolnt ut lso the pplicility of these mterils for components of experimentl devices for simulting VHTR coolnt conditions (e.g. helium loops), which were plnned to e uilt in Řež in Czech Repulic. One helium loop ws lredy uilt in [3], two more helium loops re plnned to e uilt within SUSEN project [4]. Steels P91 nd 316 re primry specified for usge t lower tempertures (up to ~55 6 C) ecuse of worse mechnicl properties t higher tempertures. Alloy 8 H is iron sed lloy Code qulified for use in nucler system t tempertures up to 76 C [2, 5]. This lloy is proposed s structurl mteril of rector internl, piping nd lso intermedite het exchnger (IHX) nd lso ws the primry high temperture lloy used in the Germn HTR progrm. The corrosion resistnce nd mechnicl properties t very high temperture of Alloy 8 H were found to e worse compred with high-nickel lloys (e.g. Inconel 617, Hstelloy X, etc.). High temperture corrosion in helium coolnt tkes plce y mechnism of interction of impurities contined in helium nd selected components of lloys (especilly Cr, Al or Ti) [2, 5, 7]. These components rect with impurities typicl for VHTR helium coolnt especilly H 2, H 2 O, CO,

2 CO 2, CH 4. The mechnism of corrosion depends on prtil pressures of prticulr components see e.g. [7] for detils. II. EXPERIMENTAL II.A. Tested metllic lloys Specimens were prepred in cse of steel P91 from roundlog with dimeter 91 mm, in cse of steel 316 SS from tht with dimeter 3 mm, for preprtion of specimens of Alloy 8 H the welded plte of Alloy 8 H (dimensions c. 15X31X1 mm) ws used. The plte ws cut from WIG-welded plte (dimension: 5X15X16 mm, filler mteril: Nicrofer S72 FM 82 (Filler Metl Tle 1: Chemicl composition of steel 316 (wt. %) Proceedings of the HTR )) [6]. The surfce of specimens ws grounded on grinder. Chemicl compositions of tested lloys is listed in Tle 1 - Tle 3. The dimensions of specimens were 1X1X5 mm, 4X5X2 mm, 4X1X2 mm nd 16.4X8.8X2 mm (used for tests in HTHL only). Some corrosion coupons 4X5X2 mm of Alloy 8 H contined weld metl nd het ffected zone metl. Specimens 1X1X5 mm (for hrdness nd micro hrdness testing) were mde of se metl, het ffected zone metl nd weld metl seprtely. Before exposure the specimens were rrnged in specimen holder. Element C Si Mn P S Cr Mo Ni Co N Fe Min Bl. Mx Bl. Tle 2: Chemicl composition steel P91 (wt. %) Element C S Mn Si P Cu Ni Cr Mo V Element Ti W Co N As S Sn Al N Fe.7 <.5 < Bl Tle 3: Chemicl composition of lloy 8H (wt. %) Element C S Cr Ni Mn Si Ti N Cu Fe.6 < R46.7 Element P Al Co II.B. Used experimentl devices nd techniques The specimens were exposed in impure helium simulting HTR helium coolnt. Before exposure the specimens were degresed in ethnol nd cetone using the ultrsonic th, thn weighed with ccurcy of.1 mg, rrnged to the specimen holder. For exposure two devices were used: High Temperture Helium Furnce (HTF) nd High Temperture Helium Loop (HTHL). HTF is n experimentl device consisting of tue furnce with ccurte djusting of temperture, close crystl retort mintining djusted gs tmosphere nd system of gs pipes with sensor of gs moisture monitoring locted in the inlet to the retort. Source of gs is usully pressure vessel with premixed gs mixture with ccurte composition. The mximum temperture in retort is 9 C, gs flow 1 l.min -1, nd tmospheric gs pressure. HTHL is n experimentl device simulting VHTR helium coolnt designed for mteril tests nd coolnt chemistry investigtion. The design of device ws presented on HTR conference in 21, ut some less chnges hve een mde. See [3] for detils. After exposure specimens were ccurtely weighed, the surfce ws oserved y SEM-EDX, some specimens were cross cut for nlysis of composition of su-surfce lyers nd for microstructure chnges oservtion. The tests of hrdness nd micro-hrdness were lso crried out using the Vickers indenter. Hrdness ws tested t strin of 3 kg, micro hrdness ws evluted using strin of 3 g. Dwell time ws 2 s.

3 mss chnge (mg.cm -2 ) II.C. Exposure of specimens In HTF the specimens were exposed t 76 C in impure helium tmosphere (see composition in Tle 4) up to 15 hours. Incresing nd decresing of temperture t the strt nd the end of ech period of the experiment ws 1 C.min -1. HTHL is up to now in so-clled test opertion. Tht mens, tht the oject of the test is the device itself. During this period of opertion the functions nd prmeters of device were tested nd relevnt reprtion mde. Even during test opertion the specimens were present in the test section of the HTHL. The prmeters during test were not constnt (see grph in the next chpter), mximum temperture reched pprox. 75 C. During this period of test opertion the loop ws filled with helium (purity 4.6), the moisture concentrtion vried during experiment (see grph elow). No further dmixtures were dded to circulting gs. Tle 4: Chemicl composition of gseous mixture Component concentrtion [vppm] Prtil pressure [P] H CO 5 5 H Helium Bl. Bl. III RESULTS AND DISCUSSION III.A. Specimens exposed in High Temperture Furnce Concentrtion of moisture in inlet gs got round 1 vppm. Figure 1 shows the specimens of Alloy 8 H in s-received stte, Figure 2 shows specimens of Alloy 8 H exposed 15 hours t 76 C in impure helium tmosphere. After exposure, the different chrcter of corrosion of weld metl nd se metl ws evident t the first sight (see Figure 3). Specimens of steels P91 nd 316 were exposed mx. 1 hours. Proceedings of the HTR 214 Figure 2: Specimens of Alloy 8 H fter exposure of 15 hours Figure 3: Specimen fter exposure for 15h - chrcters of corrosion t BM, HAZ, nd The mss chnge depending on time of exposure is presented in Figure 4. Compred to the chnge of ustenitic steel 316 nd lso ferritic steel P91 the mss gin of lloy 8 H ws higher. The weigh gin of mixed specimens (contining, BM nd HAZ) ws it lower thn this of specimens of se metl only.,5,4,3,2,1 BM 8 H BM 8 H BM+HAZ+ SS 316 P91 H exposure time (hours) Figure 1: Specimens of Alloy 8 H in s received stte Figure 4: Mss chnges of specimens of lloy 8 H fter exposure in impure helium t 76 C compred to this of ustenitic steel 316 nd ferritic steel P91 SEM pictures of of se metl (BM) smple surfces in initil stte nd fter exposure re showed in Figure 5. The results of surfce nlysis specimens in originl stte nd fter exposure re

4 Proceedings of the HTR 214 summrized in Tle 5. The smple surfces were investigted t five positions, in the tle the verge vlues re noted. With incresing time of exposure the iron concentrtion in the corrosion lyer decreses, s cn e lerned from Tle 5. The corrosive lyer is formed for the most prt from chrome compounds. The results from surfce nlysis of welded metl () nd het ffected zone (HAZ) re represented in Tle 6 nd Tle 7. It cn e oserve tht, e.g. on the surfce from welded metl low concentrtion of chrome nd enriched content of mngnese ws mesured in contrst to the initil stte of the mteril nd in comprison with het ffected zone. The surfces of the het ffected zone nd weld metl on exposed specimens could e compred on SEM pictures on Figure 6. The corrosive lyer of het ffected zone fter exposure of 15 hours is inhomogeneous, it could e cused y splling of this lyer. c Figure 5: SEM picture of smple surfce of se metl of lloy 8 H: -s received stte, -fter exposure of 1 hours, c-fter exposure of 15 hours in impure helium t 76 C. HAZ HAZ Figure 6: SEM picture of oundry of het ffected zone (HAZ) nd weld metl () of lloy 8H fter exposure of hours in impure helium t 76 C

5 Proceedings of the HTR 214 Tle 5: Anlysis of smple surfce of se metl of lloy 8H in s-received stte nd fter exposure (verge content of elements) Exposure (hours) (As received stte) Element O Al Si Ti Cr Mn Fe Ni % y tomic % % y t. % % y t. % Tle 6: Anlysis of smple surfce of het ffected zone of lloy 8 H in s-received stte nd fter exposure Exposure (hours) (sreceived stte) Element O Al Si Ti Cr Mn Fe Ni N % y t. % % y t. % % y t. %

6 Proceedings of the HTR 214 Tle 7: Anlysis of smple surfce of weld metl of lloy 8H s-received stte nd fter exposure Exposure (hours) (sreceived stte) Element O Al Si Ti Cr Mn Fe Ni N % y 2, t. % % y t. % % y t. % Microstructure on cross-section of se metl of Alloy 8 H in s-received stte consists of grins with mximum size pprox. 6 µm. Also smller grins originted likely y recrystlliztion in therml processing, unevenly extruded precipittes (MC, M 6 C) nd ornge colored inclusions (proly Ti, C, N) were oserved. After exposure of 1 hours significnt precipittion of prticles (proly M 23 C 6 nd γ ) ws oserved. On the surfce thin, compct nd spordiclly non-uniform out 2.5 µm thick corrosive lyer ws oserved. Under the corrosive lyer ~2 µm undersurfce lyer without precipittes ws oserved, corrosive ttck interfere with mteril 5 7 µm deep under the corrosive lyer. After exposure of 15 hours in impure helium t 76 C significnt precipittion of crides (proly M 23 C 6 nd γ) ws oserved. The undersurfce lyer without precipittes ws out 2 µm nd corrosive ttck interfere with mteril 5 7 µm deep under the corrosive lyer ws lso oserved. The thickness of non-uniform surfce corrosive lyer ws out 3µm. For comprison of microstructure of se metl of Alloy 8 H in sreceived stte nd fter exposure see Figure 7. On the cross-section of specimens contining weld metl the prtilly-melted zone ws oserved. After exposure the precipittion of prticles were oserved nd lso the undersurfce lyer without precipittes out 2 µm nd corrosive ttck interfere with mteril 5 7 µm deep under the corrosive lyer ws lso oserved s in cse of se metl specimens. The microstructure of cross section of the interfce of weld metl nd het ffected zone in s-received stte nd fter exposure in impure helium t 76 C re compred on Figure 8. SEM pictures of surfces of steels P91 nd 316 fter exposure of 1 hours re on Figure 9 nd Figure 1. Averge chemicl composition of surfce lyer is listed in Tle 8. The surfce lyer consists of minly compounds (proly oxides) of chromium nd mngnese. The micro structure of steel P91 on the crosssection in the initil stte is typicl for nneled mrtensitic-ferritic steel. The structure is formed y cride-ferritic mixture, wheret cride (most likely M 23 C 6 ) re seprted minly t orders of mrtensitic mteril, in smll mount cn e found proeutectoid ferrite, too. The oundry of ustenitic grins ws not well-defined. The few inclusions hve n ovl shpe. After exposure t 76 C for 1h the orderline of primry ustenitic mteril ws much more ccentuted due to seprtion of prticles (crides). It lso cme to roughening of prticles t the grin oundry nd the mteril structure. Proeutectoid ferrite ws not detected. Figure 11 illustrtes chnges in micro structure of steel P91. The micro structure of steel 316 in the initil stte is formed y lmost equixed ustenitic grin with numerous twins high content of δ-ferrite, elongted in the direction of shping. It ws shown tht within the structure cn e found thin, ovl, nd elongted intrusions, too. After exposure t 76 C nd 1h considerly decomposition of δ- ferrite tkes plce t prticles nd secondry ustenite. It ws further oserved seprtion of rough prticles, minly t the grin orders. Ares of segregtion re not visile. Seprted prticles hve severl shdes of grey nd it cn e expected tht they consist of vrious kinds of crides (M 23 C 6, M 6 C) nd intermetllic phses (,, ).

7 Proceedings of the HTR 214 Figure 12 shows the chnges in micro structure of steel 316. On surfce of specimens of steels P91 nd SS316 thin corrosion lyer out 2 µm ws oserved fter 1 hours of exposure in impure helium. HAZ HAZ Figure 8: Microstructure of interfce of weld metl nd het ffected zone of lloy 8 H: -in sreceived stte, -fter exposure of 15 hours in impure helium t 76 C Figure 7: Microstructure of se metl of Alloy 8 H on the cross-section of specimens: in sreceived stte, -fter exposure of 1 hours, c- fter exposure of 15 hours in impure helium t 76 C. c Figure 9: SEM picture of surfce of steel P91 fter exposure of 1 hours in impure helium t 76 C

8 Proceedings of the HTR 214 The vlues of hrdness nd microhrdness of tested mterils depending on exposure time re shown in grphs on Figure 13. Reltively significnt decrese of hrness ws recorded in cse of steel P91. Hrdness of se metl of Alloy 8 H incresed with exposure in contrst of hrdness of weld metl nd het ffected zone metl of this lloy, which decresed. It could men lso difference in other mechnicl properties of these lloys. The trend of micro hrdness of se metl of Alloy 8 H depending on exposure time is reverse thn tht of hrdness (micro hrdness decreses, hrdness increses with exposure time). Hrdness nd micro hrdness of SS 316 efore nd fter exposure is lmost constnt. Figure 1: SEM picture of surfce of steel 316 fter exposure of 1 hours in impure helium t 76 C Tle 8: Anlysis of smple surfce of steels P91 nd SS 316 fter exposure of 1 hours in impure helium t 76 C Steel Element O Si Cr Mn Fe Ni V P91 % y t. % SS316 % y t. % Figure 11: Microstructure of steel P91 - s-received stte, - fter exposure in He 76 C/1h

9 Gs flow (g.s -1 ) Gs pressure (MP) Temperture ( C) Hrdness (HV3) Micro hrdness (MHV 3) Proceedings of the HTR 214 Figure 12: Microstructure of steel s-received stte, - fter exposure in He 76 C/1h H BM 8H HAZ 8H SS316 P Exposure time (hours) H BM 8H HAZ 8H SS316 P Exposure time (hours) Figure 13: - Hrdness, -microhrdness of tested lloys depending on exposure time in impure helium t 76 C III.B. Specimens exposed in High Temperture Helium Loop specimen exposed in HTHL contined higher content of iron nd nickel (s could e compred with vlues in Tle 5). The vrying prmeters during test opertion of the loop re shown in the grph in Figure 14. This period of test opertion lsted out 264 hours with severl interruptions of opertion. The concentrtion of residul moisture in the gs entering nd escping the test section with specimens during opertion is shown in grph in Figure 15. In time of pulishing of this pper only one specimen of Alloy 8 H ws evluted fter exposure in HTHL. After exposure in HTHL the corrosive lyer on the surfce of the specimen ws evident (see Figure 16). The SEM picture tken from the middle of the specimen is shown in Figure 17. The results of SEM nlysis (verge vlues from nlysis of 5 sites) re listed in Tle 9. In contrst to specimens of Alloy 8 H exposed in HTF in impure helium, the surfce lyer of Temperture Gs flow Gs pressure Time (hours) Figure 14: Prmeters during test opertion of High Temperture Helium Loop

10 H 2 O (vppm) Proceedings of the HTR , 5, 1, 15, 2, 25, 3, Time (hours) Inlet AK Outlet AK Figure 15: Moisture concentrtion in circulting gs in the inlet nd the outlet to the test section of HTHL Microstructure of metl ws evluted on the logitudl section of specimen. The microstructure ws similr to tht oserved in specimens exposed in HTF. Precipittes were oserved on the grin oundries nd lso in grins. The thickness of corrosive lyer ws found to e 4.3±,7 µm. It is more thn ws found in cse of specimens exposed in HTF in spite temperture in HTHL ws during most time of exposure lower nd the exposure time ws shorter. Moreover, gs in loop contined only residul moisture (proility of presence of trces of residul ir << 1 vppm cnnot e excluded) nd no other dmixtures were dded. The cuse of this could e much higher pressure nd gs flow in HTHL thn in HTF. Mss gin of specimen fter exposure ws.6 mg.cm -2. On the other hnd mss gin of specimen fter exposure ws.6 mg.cm -2, which is less thn fter exposure in HTF 76 C/1 hours. Figure 16:Specimen of se metl of Alloy 8H fter 26 hours exposure in HTHL Figure 18: Microstructure of Alloy 8H exposed in HTHL on longitudl section Figure 17: SEM picture of specimen surfce of Alloy 8H fter exposure in HTHL Tle 9: SEM nlysis of specimen surfce fter exposure in HTHL Element O Al Si Ti % y Element Cr Mn Fe Ni % y III CONCLUSION Specimens of steels P91 nd 316 nd Alloy 8H (nd weld metl mde of Nicrofer) were exposed in impure helium t temperture up to 76 C. For exposure two devices were used High Temperture Furnce, which llows tests in low pressure nd gs flow rte, nd High Temperture Helium Loop, which cn simulte lmost rel conditions of VHTR helium coolnt, especilly high pressure nd gs flow rte. After exposure the chnges of specimens were evluted grvimetry, tests of hrdness nd micro hrdness, SEM nlysis of surfce corrosive lyer, microstructure oservtion nd mesurement of thickness of corrosive lyer on cross-section were crried out. The chnges of tested mterils were determined. According to grvimetry the most susceptile of tested lloys to corrosion in impure helium ws Alloy 8H. Fll of hrdness nd microhrdness fter exposure ws recorded in cse of P91,

11 Proceedings of the HTR 214 Nicrofer weld metl nd Alloy 8 H het ffected zone, hrdness of se metl of Alloy 8 H slightly increses fter exposure. Hrdness of SS316 ws lmost constnt. The corrosive lyer on Alloy 8 H fter 26 hours exposure in HTHL ws found to e thicker thn tht fter exposure in HTF fter 76 C/15 hours, ut the mss gin of specimen fter exposure in HTHL ws less thn tht fter exposure in HTF 76 C/1 hours. Testing of mterils nd evlution of exposed specimens still continues. ACKNOWLEDGEMENT The presented work ws finncilly supported y the TACR Alf Project TA31849 nd TA3285. The presented work ws finncilly supported y the SUSEN Project CZ.1.5/2.1./3.18 relized in the frmework of the Europen Regionl Development Fund (ERDF). The presented work ws lso supported within FP7 ARCHER project supported y Europen Commission. Authors lso thnks collegues from UJV Rez for evlution of surfces of specimens (4X1X2 mm specimens only) y SEM/EDX. REFERENCES [1] cit. 1/7/214 [2] Ntesn K., Purohit, A., Tm S. W.: report NUREG/CR-6824: Mterils Behvior in HTGR Environments, Office of Nucler Regultory Reserch, Wshington, 23 [3] J. Berk, J. Mtěch, M. Černý, I. Víden, F. Sus, P. Hájek: Nucler Engineering nd Design 251 (212) [4] cit. 1/7/214 [5] R. Wright, J. Wright, C. Cet: Comprehensive Nucler Mterils 5, (212), [6] Dr. Kloewer: Procurement of test mteril, report of ARCHER project, 211 [7] C. Cet, A. Terlin, A. Girrdin, D. Kczorowski, M. Blt, J. L. Sérn, S. Duiez Le Golf: Benchmrk CEA AREVA NP EDF of the Corrosion Fcilities for VHTR Proceedings of ICAPP 27 Nice, Frnce, My 13-18, 27, Pper 7192